Airplanes leave white trails when water vapour in their exhaust meets cold upper-atmosphere air and freezes into ice crystals. These condensation trails, or contrails, are human-made clouds. In dry air they disappear quickly; in humid air they can persist and spread into thin cirrus-like clouds.
An aircraft can seed a cloud
Jet fuel combustion produces water vapour, while engine exhaust also contains tiny particles. In the cold air at cruising altitude, vapour can condense and freeze around particles, forming the visible line. NASA explains that contrails form in much the same way as natural cirrus clouds, but start in the aircraft’s wake. [1]
Why some trails vanish and others spread
A contrail’s lifetime depends on the surrounding atmosphere. If the air is relatively dry, ice crystals shrink by sublimation and the line fades. If the air is close to ice saturation, crystals can persist, grow and spread with winds. This is why two planes at similar heights may leave trails that look very different.
What a persistent contrail becomes
Long-lived contrails can merge with other trails and add to high cloud cover. NASA studies their effects on Earth’s energy balance because clouds can reflect incoming sunlight and trap outgoing infrared radiation. The net effect depends on conditions, including time of day, altitude and cloud properties. A trail is not evidence of unusual spraying; it has a well-understood cloud-formation explanation. Compare these clouds with why ordinary clouds float.
Are contrails made of smoke?
No. Their visible white part is mainly tiny ice crystals, although exhaust particles can help provide surfaces for ice to form.
Sources and evidence
- NASA Earth Observatory — The Evolution of a Contrail ↗
NASA explains exhaust moisture, cold air, contrail persistence and cloud effects.
- NASA Earth Observatory — Curvy Contrails ↗
NASA describes contrails as ice-crystal clouds that can spread into cirrus.
Sources consulted: 11 October 2026. Written with AI assistance and checked against the linked references; not independently reviewed by a subject specialist. Sources do not endorse this article. See our editorial policy.
Revision: Initial publication; the explanation was checked against the linked institutional and research sources.